Radio amplifying circuit



Dec. 26, 1933. w 0. BARNES 1,940,991

RADIO AMPLIFYING CIRCUIT Filed May 18, 1931 Q altar-nay:

Patented Dec. 26, 1933 PATENT OFFICE RADIO' AMPLIFYING CIRCUIT William 0. Barnes, Worcester, Mass'.

` nlitil'l my 18, 1931. Seria-1 N0. 538,084

5 Claims.

means and-is an improvement on` that disclosed.

by me in patent .application Serial No. 157,122, led December 27, 1926,`patented June 23, 1931,

The objects of this invention are to provide means, in a radio amplifying circuit, whereby the feed back factor can be varied inversely with respect to the strength of the alternating current in the output circuit of the radio tube, so that at some point, within the working range of said tube, said feed back r factor shall be equal to the reciprocal of the amplification factor of the tube; to provide means whereby in the absence of an incoming signal,.the radio tube will oscillate at a predetermined potential within the working range of the tube; to yprovide the last named means in the form of a thermally sensitive'resistance and a thermally `nonsensitive resistance in the output circuit for the alternating component of the plate lament current, and to provide an arrangement whereby an inductance or a capacity can be included `in thecircuit 'to equalize the oscillating potential at .the two extremes of the working range of the tube. While my invention is applicable to both transmitting and receiving apparatus,.I have shown it as applied to a radio re- ,ceiver.`

Other objects and advantages of the invention will appear hereinafter.

Reference is toA be had to the accompanying drawing, in which Fig.,1 is a schematic diagram of a radio receiver embodying this" invention;

. 2 is a longitudinal sectional view of a preferred form'of regulating resistance used therein,

Figs. Sand ltare views similar to Fig. 1 showing modifications.

If e represents the alternating potential of a signal applied to the grid of a radio tube, E the alternating potential generated in the output circuit, m the amplificationfactor of the tube, and E/a the feed back from the output to the input circuit, :the effective amplification will be expressed by,

" E Lm e llmfl/a is variable, as it increases from zero to equality with the eective amplification factor will continuously increase to the limit w at 1/a=l/m. When l/a equals or is greater than 1 /m, the tube is in elecd trical oscillation.. When l/a is smaller but very nearly equal to l/m, the effective amplification factor is every large. With the ordinary circuits, it is difficult to work with 1 /a very close toequality with l/m, sinceY the circuit becomes, electrically, very unstable. The quantity l/a is hereafter referred to as the feed back` factor.

In Fig. l, L1 is an inductance included in the aerial circuit of a radio receiver. Coil Ll is coupled magnetically to coil L2. Coil L2 and varable condenserlCZ comprise a tuned kcircuit which is coupled in a usual manner to the grid of the radio tube 1 through a iixed condenser C5. The tube 1 may be any of the commercial varieties of radio amplifying tubes, but preferably should be of a power type. The plate of l is connected with the positive pole of battery B through a thermally sensitive variable resistance R1 and a' thermally nonsensitive resistance R2.

Resistances R1 and R2 are shunted by the tuned circuit composed of coil L3 and variable condenser C3. Since the impedance across L3C3 is very high, for an alternating current of the frequency to which it istuned, but very low for a direct current as compared with the resistances of R1 and R2, the components of the plate lament current are effectively separated. The direct component passes through L3, while the alternating component passes through Rl and R2. Resistance R2 is shunted by the tuned circuit composed of coils L4 and L4a and the variable condenser C4. Coil L40, consists of a small number of turns and is coupled magnetically with coil L2, thereby effecting a very loose coupling between the tuned circuits L2C2 and L4L4aC4. Y

The grid of detector tube 2 is connected with the plate of 1 through condenser C6. The plate of 2 is connected through loud speaker 3 with the positive pole of battery B. The filaments of tubes l and 2 are heated by current from battery A i noy mally sensitive resistance in this receiver is much greater than in the receiver shown in said patent application Serial No. 151,122, a different construction of such resistance member has been adopted. Its construction is shown in Fig. 2. A

very fine platinum wire 4 passes diagonally through the bore of a glass tube 5 and makes electrical contact with the metallic caps 6, 6 cemented to the ends of 5.

If the three tuned circuits L2C2, L3C3 and ino nest

L4L4aC4 be tuned in unison, the ratio of the alternating potential across R2 to the total alternating potential generated by tube 1 will be determined by the ratio of the resistance of R2 to the sum of the plate resistance and the resistance R1 and R2.

An alternating potential across R2 will cause an alternating current to circulate around circuit L4L4aC4. Since L4a is coupled to L2 the current in L4a. will create an alternating potential on the grid of 1 proportional to the potential' across R2. The total alternating potential on the grid of 1 is the sum of the potentials due to the incoming signal, the interelectrode capacity of the tube and the potential across R2. Hence the feed back factor is determined by the ratio of R2 to the sum of R1, R2 and the plate resistance of the tube.

The resistance R1 and R2 are, by construction, given such values that when the platinum Wire of Rlis heated by the current corresponding to a desired output potential, the feed back factor is equal to the reciprocal of the amplification factor. If in the absence of an incoming signal of the frequency to which the receiver circuits are tuned, the switch 7 is closed, tube 1 begins to oscillate. In a short interval of time the oscillatory current will heat up the wire of R1 and consequently increase its resistance to such a value that the feed back factor is equal to the reciprocal of the amplification factor. Thereafter the oscillations are maintained at a definite potential, hereafter referred toas the oscillating potential. If now a weak signal be received by L1 and impressed through L2 on the grid of 1, the resulting increase of current through R1 increases the resistance of R1 so that the feed back factor is slightly smaller than the reciprocal of the amplification factor. Hence the oscillation of the tube ceases and the signal is greatly amplified up to a mean value, only slightly in excess of the oscillating potential. If the mean strength of the signal increases to any ordinary value, the mean potential of the output is only slightly increased, for theaccompanying variation of the feed back factor Will result in a large decrease in the effective amplification. If it is desired to alter the mean strength of thev output, the adjustable shunt is moved so as to cut out more or less of the resistance of R2 from the circuit.

It is to be noted that resistance R2 may be replaced by either an inductance or a capacity. With the construction shown in Fig. 1, the oscillating potential will be greater at the high end of the frequency range of the receiver than at the low end, on account of the increased influence of the interelectrode capacity at the higher frequencies. Consequently, if the frequency range be large, it becomes desirable to equalize the oscillating potential at both extremes of the range. To accomplish this I use either of the alternative methods shown in Figs. 3 and 4.

In Fig. 3, the construction is the same as in Fig. 1 except that a capacity, condenser C7, is included in the output circuit and tuned circuit L4L4aC4 shunts both R2 and C7.

In Fig. 4 the construction is the same as in Fig. 1, except that an inductance, coil L5, is included in the output circuit. The tuned circuit L4L4aC4 shunts R2 only.

If either C7 or L5 is properly proportioned, the difference between the oscillating potentials at v both ends of the frequency range will be made as small as desired.

Although I have illustrated and described three forms of the invention I am aware of the fact that other modifications can be made therein by any person skilled in the art without departing from the scope of the invention as expressed in the claims. Therefore I do not wish to be limited in this respect otherwise than as set forth in the claims, but what I claim is:

l. In a radio amplifier comprising a radio tube and its associatedinput and output circuits, an output circuit for the alternating component of the plate filament current containing a thermally sensitive resistance and a thermally nonsensitive resistance, a coupling between the output and input circuits, said resistances and coupling constituting means whereby in the absence of an incoming signal, the tube is caused to oscillate at a predetermined potential withm the working range of the tube.

2. In a radio amplifier comprising a radio tube and its output ciiz. for the alternating component of the plate filament current containing a thermally sensitive resistance, a thermally nonsensitive resistance and a capacity, a coupling between the output and input circuits, said resistances, capacity and coupling constituting means whereby, in absence of an incoming signal, the tube is caused to oscillate at a predetermined potential within the working range of the tube.

3. In a radio amplifier comprising a radio tube and its associated input and output circuits, an output circuit for the alternating component of the plate filament current containing a thermally sensitive resistance, a thermally nonsensitive resistance and an inductance, and a coupling between the output and input circuits, said resistances, inductance and coupling constituting means whereby, in the absence of an incoming signal, the tube is caused to oscillate at a predetermined potential Within the Working range of the tube. 4

4. In a radio amplifier comprising a radio tube, an output circuit for the alternating component of the plate filament current containing a thermally sensitive resistance, and means whereby, in the absence of an incoming signal, the tube is caused to oscillate continuously and uniformly at a predetermined potential within the working range oi the tube.

5. In an electrical signal amplifying system comprising a radio tube and its associated input and output circuits, a thermally sensitive resistance and a thermally nonsensitive resistance in the output circuit, and a coupling between the output and input circuit.

WILLIAM O. BARNES.

loo

ated input and output circuits, an

ilo

iis 

